The present invention relates to a sheet feeder for use in a printer, which selectively feeds a continuous sheet and a cut sheet to a printing section in a printer.
A known sheet feeder for use in a printer comprise a platen, sheet presser rollers which can be held against and released from the platen, pin tractors for feeding a continuous sheet toward the platen, and an operating means for selecting either a continuous sheet feed mode or a cut sheet feed mode on the sheet feeder. When the operating means, such as a change-over lever, selects the continuous sheet feed mode, the sheet presser rollers are spaced away from the platen, and the pin tractors and the platen are actuated in synchronism with each other to thereby feed a continuous sheet. When the operating means selects the cut sheet feed mode, the pin tractors are inactivated and the sheet presser rollers are held against the platen. A cut sheet is fed in accordance with the rotations of the platen while being pressed by the sheet presser rollers against the platen.
In the known sheet feeder, the operating means is capable of switching between the different feed modes even during the sheet feeding operation. If the operating means is operated to select the cut sheet feed mode while the continuous sheet is being fed, then the pin tractor is stopped and the sheet presser rollers are held against the platen, with only the platen being actuated. Therefore, the continuous sheet can not be fed normally, otherwise it may be tore off, since the pin tractor serves as a load when the continuous sheet is to be fed by the platen while being pressed against the platen by the presser rollers. Conversely, if the operating means is operated to select the continuous sheet feed mode while the cut sheet is being fed, then the sheet presser rollers are separated from the platen and the pin tractors are actuated in synchronism with the platen. Consequently, the cut sheet can no longer be fed, and the continuous sheet is fed toward the platen by the pin tractor. The continuous sheet thus fed impinges upon the cut sheet, thereby causing a paper jam.
Further, in the known paper feeder, when the continuous sheet feed mode is changed to the cut sheet feed mode, or vice versa, it has been necessary that a control signal applied to the printing section or other signals associated with the sheet feed mode be changed depending upon the selected sheet feed mode. To this effect, not only the operating means but also a change-over switch provided separately from the operating means need to be manipulated when the sheet feed mode is to be changed from one to the other. The manipulations of both the operating means and the change-over switch are intricate for the operator. In addition, due to the need for providing another operating means for the change-over switch, the number of parts constituting the printer is increased and this causes to increase the manufacturing cost of the printer.
The known paper feeder is further provided with a paper bail which serves to retain a sheet of paper on the platen so that the sheet of paper supported on the platen is not released therefrom due to the resiliency. The retaining force imparted by the paper bail is constant regardless of whether the sheet of paper supported on the platen is the cut sheet or the continuous sheet.
However, in the case of using the continuous paper, the paper is mainly fed by the pin tractor and the paper bail only serves to pull the sheet so as not to be slackened. As the peripheral speed of the platen is set higher than the feeding speed by the pin tractor, the retaining force by the paper bail must be as weak as a slippage occurs between the platen and the continuous sheet. If the retaining force by the paper bail is too strong, the sheet of paper is pulled excessively, whereby a driving pulley and a driven pulley of the pin tractor are imposed undue load. As a result, the sheet of paper may be tore off or the perforations on the continuous sheet may be disengaged from the pins on the pin tractor. Therefore, the retaining force by the paper bail must be set weaker when the continuous sheet is to be fed.
On the other hand, in the case of using the cut sheet, particularly in the case of a small-size and highly resilient cut sheet, such as post card, to print the end portion of the paper, it is required that the sheet be fed only by the paper bail when the trailing end of the sheet is disengaged from the presser rollers. If the retaining force is not strong enough, the printing lines at the bottom portion of the paper tend to be displaced or the paper feeding cannot be smoothly achieved. Therefore, the retaining force must be set stronger when the cut sheet is to be fed.